JP2003304416A - Clamp circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize clamp operation. <P>SOLUTION: A sync tip clamp section 100 clamps the sync tip level of an input signal at a level V1. When the sync tip clamp section 100 stops its clamp operation, output voltage from the sync tip clamp section 100 is discharged via a discharge section 500. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clamp circuit for clamping a sync tip level of an input signal such as a composite video signal to a predetermined level, and more particularly to a clamp recovery technique when an unclamping occurs.

[0002]

2. Description of the Related Art FIG. 6 is a diagram showing a sync tip clamp section 100 which constitutes a conventional clamp circuit for clamping the level of a sync tip (horizontal sync signal) of a composite video signal. Here, the composite video signal input to the input terminal 1 is fetched through the AC coupling (coupling) capacitor C1, and the voltage V1 set by the constant voltage source 102 is input to the non-inverting input terminal and the transistor Q1.
Thus, the sync chip level of the decoded video signal is clamped to the voltage V1 and output to the output terminal 2.

That is, when the level of the input signal is higher than the voltage V1, the output of the operational amplifier 101 becomes high level, the transistor Q1 is off, and the output terminal 2 receives the signal input to the input terminal 1 from the coupling capacitor. C1
However, when the level of the input signal becomes lower than the voltage V1, the output of the operational amplifier 101 becomes lower according to the level of the input signal, and accordingly, the transistor Q1 becomes conductive and the output terminal 2 Level is voltage V
It is controlled to become 1.

However, in the clamp circuit of FIG. 6,
The clamp of the sync tip may be released due to fluctuations in the DC level of the input signal, noise propagated to the input signal path, and the like. For example, as shown in FIG. 8, the clamp level V1
When the noise of the following level arrives, it is clamped at the noise level and the whole is lifted and the clamp comes off. In this case, it takes time for the sync tip level to return to the specified level and to be clamped again. Generally, the larger the capacity of the capacitor C1, the longer the recovery time.

Therefore, a clamp circuit has been proposed in which the clamp is promptly restarted when such a clamp disengagement occurs (reference: JP-A-3-235).
481, JP 2000-101869).

FIG. 7 is a diagram showing a clamp circuit provided with measures against such a clamp disconnection. Here, in addition to the sync tip clamp part 100 shown in FIG. 6, a comparison part 400 and a discharge part 500 having a hysteresis characteristic are provided. The comparison unit 400 includes a comparator 401 having a hysteresis characteristic and a constant voltage source 402 of a voltage V2 for setting comparison reference values Va and Vb (Va> Vb).
00 includes a transistor Q10.

The comparator 401 of the comparison section 400 changes the output voltage to a high level when the input signal of the non-inverting input terminal reaches the level Va, and changes the output voltage to a low level when the input signal decreases from the level Va to the level Vb. Performs hysteresis operation. The level Va is set to a level at which the maximum level of the composite video signal is exceeded, and the level Vb is set near the clamp level V1.

In this circuit, when the clamp is released and the signal at the output terminal 2 reaches the level Va, the output of the comparator 401 of the comparison section 400 changes to a high level, turning on the transistor Q10 of the discharge section 500, Since the voltage of the output terminal 2 is discharged, its output level is lowered. When the output level drops to level Vb, the comparator 4
01 output changes to the low level and transistor Q10
Turns off and the discharge stops. Even if the clamp is released and the signal level of the output terminal 2 rises abnormally in this way, the discharge unit 500 operates and the clamp is quickly restarted.

[0009]

However, in the clamp circuit of FIG. 7, even if the clamp is disengaged, the output terminal 2
If the voltage of 2 does not reach the level Va, the discharge part 500 does not operate at all. That is, even if the clamp is released due to external noise or the like, the voltage at the output terminal 2 is not always raised to the level Va. Such a situation is likely to occur especially when the level of the input video signal is low. This state is shown in FIG. In such a case, the recovery time until the normal clamping is performed is the same as in the case of the clamp circuit of FIG.

The level Va is set in consideration of the case where a signal having a level higher than that of a normal composite video signal is input. In other words, upward noise (Fig.
0) and the image duplication prevention pulse have a large level, and it is necessary to consider that the discharge unit 500 does not operate due to these.

Therefore, it is conceivable to limit the dynamic range of the input signal to the level Va or less, or to make the level Va higher than the level of the upward noise or the image duplication prevention pulse, but the former limits the input signal amplitude. Therefore, the degree of freedom in circuit design is reduced, and the latter causes the discharge unit 500
Will not work anymore and the effect of shortening the recovery time will be lost.

The present invention has been made in view of the above points, and an object of the present invention is to provide a clamp circuit which controls a discharge part in accordance with a clamp operation so that a stable clamp operation is performed. Is to provide.

[0013]

According to a first aspect of the present invention, there is provided a sync tip clamp section for clamping the sync tip level of an input signal to a predetermined level, and a discharge section for discharging an output voltage of the sync tip clamp section. A clamp circuit is provided, further comprising control means for operating the discharge part when the sync tip clamp part stops the clamp operation for a predetermined time or longer.

According to a second aspect of the present invention, in the first aspect of the present invention, the control means includes a clamp operation detecting section for detecting a clamp operation of the sync tip clamp section, and a constant current charged clamp operation detecting section. Discharges when the clamp operation is detected, and when the charging voltage of the charging / discharging unit reaches a first level, the discharging unit is operated and the charging voltage of the charging unit is lower than the first level. The clamp circuit is characterized by further comprising: a comparator section that stops the operation of the discharge section when the voltage drops to the second level.

[0015]

1 is a block diagram showing an embodiment of a clamp circuit according to the present invention. In this embodiment, the discharge is controlled by detecting the clamp operation for the input signal. 1 is an input terminal, 2 is an output terminal, and C1 is an AC coupling capacitor. Reference numeral 100 is a sync chip clamp unit for clamping the sync chip level of the composite video signal to the level V1, and 200 is the sync chip clamp unit 100.
When the clamp operation is performed, the clamp operation detection unit detects the clamp operation, 300 is always charged with a constant current and the discharge operation is performed when the clamp operation detection unit 200 detects the clamp operation, and 400 is an output signal of the charge / discharge unit 300. Is a comparing unit for comparing with the reference value, and 500 is a discharging unit for discharging the voltage of the output terminal 2 according to the comparison result of the comparing unit 400. Clamp operation detection unit 200, charge / discharge unit 300, and comparison unit 400
Constitutes a control means for controlling the discharge part 500.

FIG. 2 is a circuit diagram of a clamp circuit embodying the respective parts. The sync tip clamp unit 100 includes an operational amplifier 101 and a voltage V, as described with reference to FIGS.
It is composed of a constant voltage source 102 of 1 and a transistor Q1.

The clamp operation detecting section 200 includes a transistor Q2 whose base is connected to the output terminal of the operational amplifier 101 of the sync tip clamp section 100, and a transistor Q3 whose base is connected to the collector of the transistor Q2.
Consists of.

The charging / discharging unit 300 includes a constant current source 3 for the current I1.
01, the current mirror-connected transistors Q4 and Q5 for commutating the current of the constant current source 301, and the transistor Q
5 and a capacitor C2 connected in parallel to the transistor Q3.

The comparison section 400 includes transistors Q6 and Q7 which are differentially connected and a transistor Q which is current mirror connected.
8, Q9, resistors R1 and R2, and a constant voltage source 4 for voltage V2
02 and a constant current source 403 for the current I2. Transistors Q6 to Q9, resistors R1 and R2, and constant current source 403
Composes the comparator 401 shown in FIG. 7 having a hysteresis characteristic of inverting at levels Va and Vb (Va> Vb).

The discharging section 500 comprises a transistor Q10 and is controlled by the output of the comparing section 400.

Next, the operation will be described. When the sync tip clamp unit 100 is performing a normal clamping operation, the transistors Q1 and Q2 are conductive only during the sync tip period, and the transistor Q3 is also conductive. In the charging / discharging unit 300, the current of the current source 301 is diverted to the transistor Q5 and the charging of the capacitor C2 is continued, but when the transistor Q3 becomes conductive, the charge of the capacitor C2 is discharged. Therefore, the capacitor C
2 is discharged during the clamp operation (sync tip period), and charged otherwise.

The voltage Vc2 of the capacitor C2 is calculated by the comparison unit 4
00 is input to the base of the transistor Q7, and when the capacitor C2 is periodically discharged each time the sync tip arrives, the voltage Vc2 becomes a voltage that changes to a sawtooth waveform and is set in the comparison unit 400. It does not reach the level Va (“state 1” in FIGS. 3 and 4).

However, when noise of a level equal to or lower than the sync tip level V1 arrives at time t1, the level of the composite video signal rises as a whole (“state 2” in FIG. 3), during which sync tip clamping is not performed. Therefore, the capacitor C2 is not discharged and its voltage V
c2 continuously increases and exceeds the level Va of the comparison unit 400. As a result, the comparison unit 400 determines that the level V
When a is detected, the output voltage (collector voltage of the transistor Q6) is inverted from the low level to the high level, whereby the transistor Q10 of the discharge section 500 is turned on and the output terminal 2 (capacitor C1) is discharged. Comparison unit 400
When the level Va is detected, the comparison reference value is set to the level Vb.
Switch to.

Even if the discharge of the output terminal 2 is started, it may take some time for the voltage to reach the clamp level V1 or lower. After reaching the level V1, the transistor Q1 becomes conductive. The clamp operation is performed, and at the same time, the transistors Q2 and Q3 are turned on and the capacitor C2 is discharged. FIG. 10 shows an enlarged view of the "state 3" and "state 4" portions of the voltage Vc2 of the capacitor C2 in FIG. "State 3" is the delayed part, and "state 4"
Is the discharge part.

When the voltage Vc2 of the capacitor C2 drops to the level Vb due to the discharge of the capacitor C2, the output voltage of the comparison section 400 returns to the low level, and the discharge section 50 returns.
The 0 transistor Q10 is turned off, and the discharge is stopped (“state 5” in FIGS. 3, 4, and 5). Further, the comparison reference value of the comparison unit 400 returns to the level Va and enters the standby state for the next unclamping.

The recovery time T from the release of the clamp to the re-application of the clamp is "state 2" in FIGS.
Is almost equal to the time from to "state 4". This time T
Is shortened by increasing the current value I1 of the constant current source 301, the recovery time can be set arbitrarily.

As described above, in the present embodiment, the electric charge of the capacitor C2 that is charged with a constant current is discharged every time the sync tip clamping operation is performed, so long as normal sync tip clamping is performed. Capacitor C2
Is periodically discharged so that the voltage Vc2 does not reach the level Va of the comparison section 400, and if the clamp is disengaged,
Preventing the capacitor C2 from being discharged periodically, the voltage Vc2 of the capacitor C2 is made to reach the level Va, and the output terminal 2
The voltage of is discharged.

For this reason, even if the unclamping occurs when the level of the input composite video signal is low, for example, near the pedestal level, it can be accurately detected and immediately re-clamped. The level Va of the comparison unit 400 is the dynamic range of the input composite video signal,
It can be set irrespective of the level of the upward noise, the level of the image duplication prevention pulse, etc., and the degree of freedom in design is increased.

[0029]

As described above, the present invention detects whether or not the clamp operation of the sync tip is stopped for a predetermined time or longer and restores the clamp operation, and all the conventional problems can be solved.

[Brief description of drawings]

FIG. 1 is a block diagram of a clamp circuit according to an embodiment of the present invention.

FIG. 2 is a specific circuit diagram of a clamp circuit according to the same embodiment.

FIG. 3 is a waveform diagram of a composite video signal in which a sync chip is clamped by the clamp circuit of the same embodiment.

FIG. 4 is a waveform diagram of the output voltage of the charging / discharging unit of the clamp circuit of the same embodiment.

5 is an enlarged waveform diagram of a part of the waveform diagram of FIG.

FIG. 6 is a circuit diagram of a conventional clamp circuit.

FIG. 7 is a circuit diagram of another conventional clamp circuit.

FIG. 8 is a waveform diagram of a composite video signal for explaining unclamping.

FIG. 9 is a waveform diagram of a composite video signal for explanation, which takes time to recover from the unclamping.

FIG. 10 is a waveform diagram of a composite video signal for explaining the setting of the level Va for performing recovery from unclamping.

[Explanation of symbols]

1: Input terminal, 2: Output terminal 100: Sync tip clamp part, 101: Operational amplifier, 102: Constant voltage source 200: Clamp operation detection part 300: Charge / discharge part, 301: Constant current source 400: Comparison part, 401: Comparison Vessel, 402: constant voltage source,
403: constant current source 500: discharge unit

Claims (2)

[Claims] 1. A sync tip clamp unit for clamping a sync tip level of an input signal to a predetermined level, and a discharge unit for discharging an output voltage of the sync tip clamp unit, wherein the sync tip clamp unit is for a predetermined time or more. A clamp circuit comprising control means for operating the discharge part when the clamp operation is stopped. 2. The control means according to claim 1, wherein the control means detects a clamp operation of the sync tip clamp portion, and a constant current is charged, and the control means is discharged when the clamp operation detector detects the clamp operation. A charging / discharging unit, and when the charging voltage of the charging / discharging unit reaches a first level, the discharging unit is operated, and when the charging voltage of the charging unit drops to a second level lower than the first level, the discharging A clamp circuit comprising: a comparison unit that stops the operation of the unit.